A resin composition such as a methacrylic ester polymer is produced by continuous polymerization according to which a raw material monomer, a polymerization initiator, etc., are continuously supplied to a reactor to be polymerized. A continuous solution polymerization method using a solvent (or a dispersion medium, that will also apply hereinafter) to conduct a continuous polymerization, and a continuous bulk polymerization method using no solvent to conduct a continuous polymerization are known as the continuous polymerization methods.
Generally, the continuous solution polymerization method has low productivity because of the use of a solvent, and is not efficient. In contrast, the continuous bulk polymerization method has an advantage of being able to efficiently produce a polymer composition because the polymerization reaction is conducted without using any solvent. The continuous bulk polymerization method, however, practically has various problems compared to the continuous solution polymerization method, such as that reaction control is difficult due to high viscosity of a reaction mixture, and that, when an inner face of a reactor is cooled to remove heat from the reaction system, this degrades the quality of the polymer composition and, as a result, the quality of a resin composition obtained therefrom. A method has therefore been proposed according to which, using a reactor of a complete mixing type, the reactor is fully filled with a liquid to exclude any gas phase part therefrom and continuous bulk polymerization is conducted under an adiabatic condition with no heat transfer to and from the outside (Patent Document 1). Moreover, to secure such an adiabatic condition, a continuous polymerization apparatus has been proposed that controls a supply amount of a raw material monomer and a supply amount of a polymerization initiator such that the temperature in the reactor is caused to be equal to a set temperature of an outer wall face of the reactor (Patent Document 2).